Blood is circulated through the body by the heart during its rhythmic pumping cycle, which consists of two distinct periods—systole and diastole. Heart muscle contracts to eject blood from the ventricles during the systolic period of each cardiac cycle (CC). Ejection of blood from the ventricles generates arterial blood pressure and flow adequate to deliver blood throughout the body. The blood transports oxygen, nutrients and metabolic products, removes carbon dioxide and waste, and facilitates critical physiological functions such as heat exchange. The heart subsequently relaxes during the diastolic period of the CC, when the atrial and ventricular chambers refill with blood in preparation for the heart's next contraction.
Unlike the rest of the body, which receives most of its blood flow during the systolic portion of the arterial pressure cycle, contraction of the heart during systole generates high forces within the heart's muscular walls, preventing blood from flowing through the heart muscle itself at that time. Therefore, the heart's own arterial blood supply is delivered primarily during diastole, when the heart muscle is relaxing and the heart chambers are filling for the next contraction, while at the same time the lower residual blood pressure in the aorta pushes blood through the coronary arteries and into the myocardial muscle to supply the heart with its needed oxygen and nutrients.
In addition to the heart's pumping function, the musculoskeletal (MSK) system also pumps arterial and venous blood throughout the body during physical activity in a couple of important ways. First, skeletal muscle contraction and relaxation cycles during rhythmic physical activities cause regular oscillations in peripheral arterial and venous blood pressure or flow due to intermittent compression of the vasculature that travels within, between, and adjacent to the skeletal muscles. Second, MSK movement can lead to periodic acceleration and deceleration of the intravascular volume of blood against gravity and inertia.
When rhythmic muscle contractions or MSK movements are favorably coordinated with the timing of the heart's pump cycle, the MSK and cardiac pumping systems can augment one another to increase blood flow to and perfusion of important areas of the body with less pumping energy expended by the heart. This favorable coordination of these two pumping systems can be referred to as “musculoskeletal counterpulsation” (MCP). During MCP, maximum rhythmic MSK-induced blood pumping consistently increases central arterial blood pressure when the heart is relaxing and refilling between contractions (i.e. during diastole), and the maximum cardiac induced pumping (systole) consistently occurs between MSK induced maximal central arterial pressure events. On the other hand, when rhythmic muscle contractions and MSK movements occur with uncoordinated, or worse, unfavorably coordinated timing, blood flow and perfusion are decreased along with a concurrent decrease in pumping efficiencies. Unfavorable coordination occurs, for example, when the cardiac and MSK systems consistently pump blood maximally into the central circulation at substantially the same time during rhythmic physical activity. This unfavorable coordination of the two pumping systems can be referred to as “inverse musculoskeletal counterpulsation” (iMCP).
Typically, when individuals walk, run, bicycle, or participate in any rhythmic physical activity, most experience favorable coordination between MSK blood pumping and CC blood pumping only intermittently. Even when an individual's heart rate (HR) and MSK activity cycle rate (MSKR) happen to be substantially equal, the respective timing of the two pumping systems may result in favorable or unfavorable coordination, or somewhere in between. A certain degree of “cardio-locomotor synchronization” can occur during rhythmic physical activity, in which the timing of an individual's MSK pump cycle relative to the heart's pump cycle tends, statistically, to naturally favor MCP. However, when such synchrony does occur, it is usually only a temporary phenomenon since HR and/or MSKR can change as environmental factors vary (e.g., running in hilly terrain or variable wind) or with any of several physical changes, such as alterations in effort, speed, hydration, temperature, catecholamine levels, or fatigue.
The benefits of favorable coordination between MSK movements and the heart's pump cycle can include improved perfusion and oxygenation of cardiac and peripheral skeletal muscle and possibly other tissues; decreased HR due to increased cardiac preload and stroke volume; decreased systolic blood pressure and pulse pressure; decreased required respiratory effort to meet decreased oxygen demands; and reduced muscle fatigue due to improved skeletal muscle perfusion. These benefits can potentially lead to increased physiological efficiency, decreased myocardial stress, increased aerobic energy production, improved aerobic fat metabolism, enhanced individual performance, and a potential increase in the health benefits and safety of rhythmic physical activity. Conversely, unfavorable coordination between MSK movements and the heart's pump cycle can lead to the opposite of all of these effects.
Some of the general approaches that we have described for favorably coordinating MSKC and CC timing during rhythmic physical activity include (1) the provision of adaptive real-time MSKC timing prompts to a user; (2) automated means of adjusting exercise equipment settings in order to adaptively modify a user's MSKC timing; and (3) automated means of adjusting artificial cardiac pacemaker systems to adaptively adjust the timing of the CC relative to the MSKC of the user. Each of these general approaches may require the identification and use of sensed physiological metrics to assist with identifying a target timing relationship between the MSKC and CC of the user, measuring physiological impacts of the timing relationship, and tracking progress in favorably influencing physiology over time.
The methods and systems described below are for guiding a user to obtain and maintain favorable coordination of MSKC and CC hemodynamics, and more directly, to achieve or maintain system calibration, to increase the accuracy of identifying, achieving, and maintaining target pump timing relationships, and/or to track the effectiveness of achieving physiological benefit during rhythmic physical activity.